Switching systems such as the telephone switching network are generally designed as point-to-point networks to interconnect, upon request, selected pairs of customer terminals from a large plurality of terminals connected to the system. The simplest connecting network capable of interconnecting N.sub.1 input terminals and N.sub.2 output terminals is a rectangular N.sub.1 .times.N.sub.2 array of switching elements or crosspoints. Although such a rectangular array is non-blocking in that any two idle customer terminals are always connectible regardless of the array interconnection of other terminals, the rectangular array is not a practical network in most applications due to the prohibitive cost of the large number of array crosspoints.
Significant crosspoint cost reductions are obtained by designing networks having small blocking probabilities determined to be acceptable in many applications. However, in other applications blocking networks are not acceptable. For example, customers that are provided with a large selection of video signals via a switching network might reasonably consider the network blocking of their favorite channels as unacceptable. At the same time, the crosspoint cost for such a high-frequency switching network becomes an even more important consideration.
One known non-blocking network having significantly fewer crosspoints than a rectangular array is disclosed in an article by C. Clos, "A Study of Non-Blocking Switching Networks," Bell System Technical Journal, March 1953, pages 406-424. The network, referred to herein as the three-stage Clos network, comprises r.sub.1 rectangular n.sub.1 .times.m first stage switches, m rectangular r.sub.1 .times.r.sub.2 second stage switches and r.sub.2 rectangular m.times.n.sub.2 third stage switches. There is exactly one link connecting each first stage switch to each second stage switch and one link connecting each second stage switch to each third stage switch. A three-stage Clos network wherein the number, m, of second stage switches is given by EQU m=n.sub.1 +n.sub.2 -1,
in a non-blocking, point-to-point network. This is true since a given first stage switch input terminal is always connectible to a given third stage switch output terminal via a second stage switch that is connected to none of the other n.sub.1 -1 input terminals of the given first stage switch and none of the other n.sub.2 -1 output terminals of the given third stage switch. However, a three-stage Clos network having the number, m, of second stage switches given by the above equation is not a non-blocking multiconnection network as is illustrated later herein by an example. The example involves a multiconnection network that is referred to as a broadcast network since any given network input terminal is connectible to any or all output terminals.
As is discussed in the article by F. K. Hwang, "Three-stage Multiconnection Networks Which Are Non-blocking in the Wide-Sense," Bell System Technical Journal, Vol. 58, no, 10, December 1979, pages 2183-2187, three-stage multiconnection Clos networks have been designed which are non-blocking in the wide sense, i.e., non-blocking when a particular connection strategy is followed, by providing a significantly increased number of second stage switches. Again, however, the large crosspoint cost associated with the increased number of second stage switches makes such a three-stage Clos multiconnection network an extremely expensive alternative for switching networks serving even a modest number of customer facilities.
It is possible for a customer facility connected to a multistage switching network to occasionally be blocked from being connected as desired because the network happens to be interconnected in a manner that prevents effecting the desired interconnection. This, of course, is an undesirable situation which, in an appropriately designed network, is remedied by dismantling one or more existing interconnections and rearranging the interconnection paths to accommodate the new request. When such a rearrangement is possible, it is said that the new assignment, which is the new set of interconnections desired to be established, is realizable. A switching network which can realize all possible assignments without rearranging existing connections is said to be non-blocking, while a network which can realize all possible assignments only by occasionally rearranging existing connections is said to be rearrangeable. Typical rearrangeable networks have many less crosspoints than their non-blocking counterparts. An illustrative rearrangeable network, along with the common control equipment associated therewith, is disclosed in U.S. Pat. No. 3,129,407 issued to M. C. Paull on Apr. 14, 1964. Other rearrangeable networks are disclosed in the article by V. E. Benes, "On Rearrangeable Three-Stage Connecting Networks," Bell System Technical Journal, Vol. 41, no. 5, September 1962, pages 1481-1492 and in U.S. Pat. No. 4,038,638 issued to F. K. Hwang on July 26, 1977. Each of these known switching networks is, however, a rearrangeable point-to-point network rather than a rearrangeable multiconnection network. Further, each of these networks comprises three or more stages of switching. In applications where network distortion and delay parameters directly related to the number of crosspoints required to effect a given connection are important, the transmission quality obtainable through such three-stage networks is therefore limited.
In view of the foregoing, a recognized problem in the art is that costly multiconnection networks which are non-blocking without rearrangement must presently be used in applications where blocking is unacceptable and high transmission quality is required since the known rearrangeable networks are only point-to-point networks and comprise at least three switching stages.